ADSL transmission systems require low pass filters at the customer end to prevent high frequency ADSL signals from interfering with the POTS service and also to prevent HF transients from the POTS interfering with the ADSL transmission.
In the context of a telephone line carrying both ADSL & POTS signals, it has been found that multiple in-line filters on a single line cause progressive degradation of return loss at the POTS/Line ports.
Australian Application No. 36813/99 discloses the use of a switched capacitor to change the filter characteristics when the phone goes off-hook and to reduce the effect of multiple filters on the one line. This may, for example change the filter from a second order filter to a fourth order filter.